Commercially available corrosion inhibitor pigments generally used for Fe, Al, or Cu protection, belong exclusively to only a few generic types of inorganic compounds, such as chromates, phosphates or polyphophates, molybdates, borates, silicates and phosphites of Zn, Ca, Sr, Ba, Al, Mg, Pb, Cr, Fe, or various combinations of these anionic and cationic species. Some of the latter cationic species, particularly Ba, Pb and Cr are known to be toxic. Transition metal derivatives of hydrogen cyanamide, particularly ZnNCN are also known for pigment grade application, limited, however, to special mirror backing coatings intended for Ag protection.
The active inhibitor species generally are the anionic constituents. The cations present, however, determine important physical properties of such pigments, i.e: solubility or kinetic availability of the effective species.
Chromates and particularly SrCrO.sub.4, (characterized by desirable combination of relatively high CrO.sub.4.sup.-2 content and optimal solubility) are regarded in the art, as being the most versatile, being compatible with all paint vehicles, highly effective on all metal substrates, and thus being effective, although toxic, pigment grade corrosion inhibitors.
It is the redox activity of chromate species, accountable for their inhibitive efficiency, which allows interference with both anodic and cathodic corrosion processes. As a consequence, chromates are considered anodic-cathodic or "mixed" type inhibitors. A direct result of Cr(VI) reduction by corrosion processes, the formation of Cr(III) species in situ of metal substrates' protective oxide layers is a distinctive feature of chromate's corrosion inhibitive mechanism. As a consequence, the protective oxide layer on Fe, for example, is assumed to be a composite of increased hydrolytic stability formed by Fe(III) and Cr(III) species.
In contrast with chromates, all the other above specified active corrosion inhibitive pigments, and more specifically the pertinent anionic constituents of phosphates, molybdates, silicates, borates, phosphites and cyanamides are "redox inactive" under normal conditions of metal corrosion. Consequently they do not qualify as electrochemically active corrosion inhibitors.
A characteristic feature of "redox inactive" pigments' inhibitive mechanism is the related anionic species' limited ability to interfere with corrosion processes. It is assumed that "redox inactive" inhibitive species are active in anodic environments, presumably by a mechanism of insoluble precipitate formation, involving anodically solubilized metal cations. There are, however, significant consequences of "redox inactive" pigments' less efficient corrosion inhibitive mechanism. In this sense, generally, it can be stated, that non-chromate based pigment grade anodic corrosion inhibitors are less effective, and thus qualitatively inferior, but, however, non-toxic alternatives to the chromates. This situation also applies to diverse, multi-phase pigment systems which contain various combinations of anionic and cationic species as above disclosed.
In contemporary industrial practice pigment grade non-toxic alternatives to chromates enjoy commercial acceptance, limited practically, however, to applications intended for iron or steel protection. More specifically, it is well known that in aircraft and coil coatings, where top corrosion inhibitive performances are required, chromate pigments, and specifically SrCrO.sub.4, have no equally effective chromate-free alternatives.
All known electrochemically active pigment grade corrosion inhibitors, which represent the state of the art, display some degree of substrate specific behavior. For example, chromates and particularly SrCrO.sub.4, highly effective pigment grade inhibitors applicable practically on all metals (such as Fe, Al, Cu) are known to actually promote Ag corrosion and consequently are incompatible with such substrates.
Non-toxic alternatives to chromates such as phosphates, molybdates, borates, silicates and diverse combinations of the same, are valued, although less efficient inhibitors of Fe corrosion, which however, perform poorly on Al, Cu and are just as incompatible with Ag. Their industrial application is thus limited practically to anti-corrosive protection of Fe.
In view of these considerations, it has been widely concluded that development of equally effective, non-toxic inorganic alternatives to pigment grade chromates is not a realistic probability. Thus, a need has existed which presents a challenge to those dealing with the chemistry of corrosion inhibitors to develop such chromate-free pigment grade products, able to close the above-described performance gap, which exists between the best non-chromate based and chromate based inhibitor pigments.
The real challenges faced in inhibitor pigment development, become apparent by taking into account the general requirements imposed by the actual commercial and industrial practice (such as high inhibitive efficiency, low toxicity and environmental hazard, competitive price and versatility in application) as well as by considering the limited number of anionic and cationic species suitable for pigment synthesis.
The present invention, thus, concerns the development of multi-phase pigment systems which display synergy in respect of corrosion inhibition and are constituted essentially of various combinations of the above-mentioned limited number of qualified ionic species. It must be kept in mind, however, that synergistic behavior in respect of corrosion inhibition is not an ordinary occurrence observable with just any random mixtures of chemically different, finely divided solid constituents containing inhibitor species. A high degree of synergy, rather, is an unpredictable property of some multi-phase pigment systems of distinct chemical and phase composition, having two or typically more solid component phases, which individually are all characterized by comparatively lower corrosion inhibitive activity than the combined system itself. Notably, the active component phases of the system, preferably all insoluble in organic media, should nonetheless be water soluble to a limited but effective extent.
There are essentially three different physical states in which two (or more) microcrystalline or amorphous component phases of distinctively different chemical composition can co-exist as constituents of multi-phase and finely divided solid systems: ordinary physical mixtures, micro-composites and solid solutions.
Solid solutions, although formed spontaneously, are not the ordinary state found in multi-phased solid systems. In some cases however, distinct combinations of three or more anionic and cationic constituents, which ordinarily form two or more solid phases of distinct chemical composition, in special conditions form unified solid phases of complex chemical composition. Such unified phases are characterized by uniform distribution at molecular level of all constituent ionic species. It will be noted that significant numbers of colored pigment grade products are known to consist of solid solutions, such as chrome yellows or molybdate oranges, which are solid solutions of PbCrO.sub.4 and PbSO.sub.4, or, in addition, of PbMoO.sub.4, respectively. An applicable example is disclosed in my U.S. Pat. No. 5,558,706. Notably, solid solutions are characterized by distinct values of physical parameters such as solubility, specific gravity, etc.
Ordinary mixtures of finely divided (and normally polydispersed) solid phases of different chemical composition are constituted of distinct and separable microparticles of the distinct component phases, mixed and uniformly distributed in the system. Ordinary mixtures can be prepared by simple mechanical procedures. However, they are often formed spontaneously in chemical processes as well, such as during concurrent or subsequent formation by precipitation of two or more solid phases in aqueous systems.
Conversely, micro-composite (multi-phase and finely divided) solid systems are constituted of microparticles, containing distinctly identifiable, but physically inseparable component phases of different chemical composition which form common interfaces and are held together by chemical forces. Typically, composite microparticles possess a morphological configuration of a coating-core type, often formed in some heterogenous chemical processes, such as described in my U.S. Pat. No. 5,176,894, wherein a finely divided suspension of an essentially insoluble solid reactant is reacted with a dissolved component reagent of a liquid phase and consequently is converted into a finely divided suspension of a solid reaction product, which is insoluble in the reaction medium. During the course of the reaction the suspended solid phase consists of composite micro-particles of a coating-core configuration, in which both solid component phases, ie., the reaction product coating and the reactant core are simultaneously present, separated by an interface of approximately spherical shape. This mechanism, obviously, implies the reagent species' continuous diffusion through the coating phase toward the reactant core and generally, the reaction is diffusionally impeded. Thus, by preventing completion of the reaction, the solid product's micro-composite state is preserved. As used herein, "microparticles" are defined as particles having diameters of approximately 0.5 to 10 microns. The preferred particles are in an approximate diameter range of 1 to 5 microns.
Notably, there are also numerous examples known to the art regarding preparation of micro-composite multi-phase solid systems (consisting of micro-particles with coating-core structural morphology) which, in comparison, relate to a quite different process and mechanism. Known as "particle encapsulation" procedures, widely used in the pigment manufacturing industry, are the formation by precipitation of chemically inert coatings of diverse chemical composition on the surfaces of finely divided solids in aqueous suspension.